Room.createConstructionSite allows invalid placement
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The UI and the createConstructionSite API disagree on whether it should be possible to place a construction site in a location where it cannot be built. For example, placing an extension site on an existing spawn: cannot be done with the UI, can be done with the API.
Because createConstructionSite will return ERR_INVALID_TARGET if the structure cannot be built somewhere, I suspect it's unintended that the API can do this.